This invention relates to an apparatus and method for separately forming single drops from a fluid suspension and more particularly relates to a method and apparatus for detecting a particle in a fluid stream forming the particle in a droplet and separating that droplet from the fluid stream.
An apparatus capable of sorting minute particles in accordance with distinctive characteristics among the particles themselves has many important uses. For example, in the field of biology, it would be useful to sort blood cells in accordance with size. U.S. Pat. No. 3,380,584 issued to Mack J. Fulwyler, the applicant herein, shows and describes a method and apparatus for detecting and separating particles such as blood cells. The apparatus shown in the above-noted patent includes an orifice through which fluid containing the particles is caused to flow. The fluid is vibrated or pulsed by an accoustical device causing the fluid to jet through the orifice and form a fluid stream in air which separates into discrete equal volume droplets. As the particles pass through the orifice they are sensed, and as the droplet surrounding each particle is formed, it is charged so that the droplet and particle can be deflected for collection.
Although the above noted apparatus has allowed particle detection and separation at a rate, and in a manner previously considered impossible, certain problems have arisen.
In the apparatus described in the above noted patent, the acoustical device takes the form of a piezoelectric generator that vibrates at a 40 KHz rate. The vibrations are periodic, however, the particles passing through the apparatus are detected at random intervals. Because the pulsing is periodic and the detection is random, there is no assurance that each detected particle will be enclosed in a formed droplet. It is possible that a particle will appear at the neck between formation of two droplets. For this reason, it is necessary to charge and deflect at least two particles in series in order to ensure collection of the detected particle.
The above noted apparatus envisions the possibility of two different types of particle detection. One type of particle detection employs a laser beam which passes through the fluid stream, jetted into air, to a photodetector. As the particle passes through the laser beam, the change in laser light coupled to the photodetector is sensed thus recognizing a particle passage. For this type of detection to be feasible, the fluid surrounding the particle must move uniformly with as few disturbances as possible so that the laser beam is not disturbed in its passage to the photodetector. The 40 KHz vibration produced by the acoustical driver creates tiny disturbances on the fluid stream which are undesirable and should be eliminated to minimize disturbance of the laser beam.
The other type of detection employs two electrodes, one on either side of an aperture, and in contact with the particle laden fluid. The electrical resistance between the electrodes is affected by the presence and size of a particle passing through the aperture and this change in resistance is detected. This type of detection is somewhat sensitive to extraneous noise and the 40 kHz periodic vibrations produced by the piezoelectric generator constitutes a source of noise which can affect the detector operation.